Most people will acknowledge that the camera can be used as a powerful tool for telling lies. Harder to accept is that the camera, by its very nature, always lies, that it always misrepresents, distorts or manifests the hidden manipulation of its operator.

But it’s a truth ineluctable after spending time in the National Gallery of Art’s fascinating and provocative “I Spy: Photography and the Theater of the Street, 1938-2010.” The exhibition is devoted to six photographers who explored ways to photograph surreptitiously, or without intruding on the drama of their subjects’ private existence. In Walker Evans’s 1938-1941 “Subway Portraits,” the photographer concealed a camera in his coat and captured straight-on images of people sitting opposite him on a train. Decades later, photographer Philip-Lorca diCorcia rigged elaborate photographic traps in New York, concealing synchronized flashes and using a pre-focused telephoto lens to record spontaneous moments of street life with the professional polish of a fashion photographer.

The assumption driving these experiments is simple but problematic: By masking the presence of the photographer, one can get a deeper, more unguarded truth about people. As Evans put it, he wanted to capture people “in naked repose,” with their guard down and “the mask” off. Whether it’s Freudian slips of tongue, unwanted conversations caught on a hot mike or leaked videotape from cameras no one knew were on, we tend to believe the spontaneous self is the honest self. But it’s a quirk of modernity to believe that the social mask is false and that there is some kind of genuine authenticity underneath it.

Study the images Evans made in the subway, and it’s hard to believe that many of his subjects were unaware of his presence. Perhaps they didn’t see the camera, but several seem cognizant of something odd about his presence. It’s one thing to rig a camera to peer out between the buttons of your coat and respond to a shutter cable in your jacket; it’s another to mask your own presence in a highly artificial game. Some of his subjects might be oblivious, but at least a few might be giving him a subtle fish eye or awkwardly avoiding him.

Evans changed the equation in another series made in Bridgeport, Conn., in 1941. Standing on the street, using a larger viewfinder camera, Evans photographed passersby without hiding the camera. The faint, questioning looks one sees in the “Subway” series become a catalogue of annoyance and confrontation, as his Bridgeport subjects look sideways and down to where the camera was positioned at Evans’s waist. The angle of their gaze is the same as it would be if Evans’s fly were unzipped, or if he were engaged in some other waist-level indiscretion. Perhaps there’s humor lurking here: The voyeuristic photographer makes himself a spectacle for others.

Dismemberment

The so-called “observer effect” in physics — that merely observing a system changes it in some way — comes to mind as you travel deeper into these experiments.

In a 1950 series, Harry Callahan photographed women on the streets of Chicago. Whereas Evans kept his camera hidden, Callahan used a pre-focused telephoto lens, picking his target and snapping it before his presence was noticed. Or maybe not. The women whose faces fill the frame in Callahan’s images often look weary and slightly pained. Most likely they are tired or preoccupied. But perhaps to some extent, they are collectively registering the annoyance of being women in a world of the male gaze. They don’t respond directly to Callahan’s camera, but they wear the protective mask of women who are used to being stared at, registering existential annoyance at a world in which men play games like the one Callahan is playing.

Robert Frank, fresh from his magnum opus “The Americans,” used a bus for photographic anonymity in his 1958 “From the Bus” series. Frank’s characteristic sense of alienation pervades these images, at least two of which dismember the bodies they capture. In one, a hand dangling from the bus is disconnected from its owner, in another a man who has passed out of the picture frame has left a foot behind, a reminder of the perpetual flux of city life.

Dismemberment is a good metaphor for the effect the camera has on our view of urban life in many of these images. Most of these photographers are breaching social protocols, and the moody and anxious view of city life they come up with might be its own form of illusion. Anything ripped from context will seem disconnected and bizarre, and so it’s no surprise that they create visions of the city in which nobody seems to be in a relationship with anyone else.

Bruce Davidson’s “Subway” is a powerful but disturbing series made in the 1980s when New York was a much grittier place than the Giuliani-scrubbed and Bloomberg-curated tourist mecca of today. But it feels dated. In part, it is the subject matter: graffiti-covered subways, gang members, thugs, Guardian Angels, terrified middle-class commuters and what appears to be a mugging in ­progress. It is all documented with the precision and care of a National Geographic cover story on exotic and dangerous animals. Davidson asked permission to photograph most of his subjects, so he isn’t really “spying.” But the people in his images seem so caught up in collective degradation, it’s hard to imagine that they could foretell how lurid it would look when reproduced in Davidson’s wonderfully rich and saturated prints.

A series of photographs from 2002 and 2012 by the Swiss photographer Beat Streuli aims to breathe new life into street photography. Streuli captured random people near the entrance to a subway stop in Manhattan, then projected the still images on two high-definition screens. Some faces linger longer than others, and as they slowly fade in and out, they seem to enter into elusive but tangible relationships with each other and the photographer. Faces seen straight on indict the viewer for invading the subject’s space. In some cases, disconnected faces appear to study each other, converting the victim of voyeurism into the voyeur. Where Frank, Evans and Callahan dismembered social relationships by picking out individual people, Streuli creates a series of faux social relationships, carefully managed by the photographer’s juxtapositions.

Changing focus

By the end of the exhibition, you might wonder whether the city is quite so horrible as its street photographers seem intent on making it. Our sense of urban woe might be partially manufactured by the kinds of images these photographers make.

Three very large changes in the world will probably make this photography seem a little passe until a revolution in taste comes. First, small cameras are everywhere, and governmental and commercial surveillance is nearly complete in many cities. Capturing people unawares is easy and ubiquitous. Second, the number of snapshots circulating in the world, on social media and the Internet, has diluted not only the impact of the form but our sense of privacy as well. Many of us freely circulate, with no embarrassment, images of ourselves with our “mask off,” so much so one wonders whether the mask even exists anymore.

Finally, the city isn’t the forlorn, anxious realm of disconnected misery that it once was, or that these photographs claim it is. Much of the social despair, isolation and pathology once seen as particularly urban has gravitated out of the cities, to the lands where people bowl alone and cook meth in beat-up trailers. And not surprisingly, the dissecting gaze of the camera has turned elsewhere, so much so that this show feels like a fascinating but almost remote chapter in the history of photography.